Method of strengthening low carbon steel and product thereof

ABSTRACT

Deep drawing steel is strengthened by alloy-nitrogen precipitation strengthening to a minimum average yield strength of 50 ksi. A deoxidized, low carbon steel sheet stock or article formed therefrom, containing from about 0.02 percent to 0.2 percent titanium in solution, from about 0.025 percent to 0.3 percent columbium in solution, from about 0.025 percent to about 0.3 percent zirconium in solution, alone or in admixture, is heat treated at 1,100* - 1,350*F in an atmosphere containing ammonia in an amount insufficient, at the temperature and time involved, to permit formation of iron nitride.

United States Patent 1191 Hook 1451 Nov. 12,1974

[ METHOD OF STRENGTHENING LOW CARBON STEEL AND PRODUCT THEREOF [75] Inventor: Rollin E. Hook, Dayton, Ohio [73] Assignee: Armco Steel Corporation,

Middletown, Ohio 221 Filed: Nov. 14, 1972 21 Appl. No.: 306,390

10/1966 Shimizu et a1.

2/1967 Shimizu et a1. 148/16 3,765,874 10/1973 Elias et a1. 148/36 Primary Examiner-W, Stallard Attorney, Agent, or FirmMe1ville, Strasser, Foster &

Hoffman [57] ABSTRACT Deep drawing steel is strengthened by alloy-nitrogen precipitation strengthening to a minimum average yield strength of 50 ksi. A deoxidized, low carbon steel sheet stock or article formed therefrom, containing from about 0.02 percent to 02 percent titanium in solution, from about 0,025 percent to 0.3 percent c0- lumbium in solution, from about- 0.025 percent to about 0.3 percent zirconium in solution, alone or in admixture, is heat treated at 1,100 1,350F in an atmosphere containing ammonia in an amount insufficient, at the temperature and time involved, to permit formation of iron nitride.

21 Claims, No Drawings 1 METHOD OF STRENGTHENING LOW CARBON STEEL AND PRODUCT THEREOF BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method of strengthening stamped-or deep drawn articles after forming. Currently available high strength sheet stock cannot be extensively formed directly by stamping or deep drawing because of its limited ductility and drawability. The method of this invention involves the novel concept of producing stamped or deep drawn parts from a low strength,.deep drawing quality steel, and subsequently strengthening the parts by alloy-nitrogen precipitation strengthening. Cold rolled and annealed sheet stock can also be strengthened in the same manner before forming to attain higher yield strength than has hitherto been possible.

2. Description of the Prior Art The hardening of steel surfaces by heat treating in an ammonia-containing atmosphere to form an ironnitrogen austenitic structure which is transformed by quenching to a martensitic structure having high surface hardness, has been practiced for many years. Prior art nitriding practices are described in ASM Metals Handbook, 1948 edition, pages 697 702, and the references cited therein. Under present practice, nitriding is performed on particular types of steels (such as Nitralloy type, austenitic stainless steels, SAE and similar steels) in the machined and heat treated condition to provide great wear resistance, retention of surface hardness at elevated temperature and resistance to certain types of corrosion. Reference may also be made to U.S. Pat. No. 3,399,085 issued Aug. 27, 1968, to H. E. Knechtel and H. H. Podgurski, relating to nitriding of a Nitralloy type steel.

The nitriding of steels containing nitride-forming alloying elements is discussed in Transactions AIME, volume 150 (1942), pages 157 171, by L. S. Darken. The nitriding of iron-aluminum alloys in an ammoniahydrogen atmosphere is described in Transactions Met.

Soc. AIME, volume 245 (1969), pages 1,595 1,602 and in Transactions Met. Soc. AIME, volume 245 (1969), pages 1,603 1,608, by H. H. Podgurski et al.

A comparison of nitrided iron-aluminum alloys and iron-titanium alloys is given in Transactions Met. Soc. AIME, volume 242 (1968), pages 2,415 2,422, by V. A. Phillips and A. V. Seybolt. It was concluded in this article that an alloy containing 1 percent titanium developed substantially higher surface hardening than a 1 percent aluminum alloy due to the very small particle size of the titanium nitride which was formed, less than about Angstroms. It was suggested that the nitride particles must be within a range of about 10 to 40 Angstroms or smaller in diameter, in order to produce maximum hardening. The particle size of aluminum nitrides in the aluminum-bearing alloy was substantially coarser.

Boron, Calcium, Columbium and Zirconium in Iron and Steel, by R. A. Grange et al., John Wiley and Sons, Inc., publishers, pages 173 179, discusses columbium as an alloying element in nitriding steels. It was concluded therein that columbium readily combines with nitrogen at temperatures above 750F if present in excess of the amount required to combine with all the carbon to increase the surface hardness of the steel.

U.S. Pat. No. 3,671,334, issued June 20, 1972 to .l. H. Bucher et al, discloses a medium-carbon columbium-modified renitrogenized steel containing less than about 0.02 percent total of aluminum, zirconium, vanadium and titanium. Sufficient free nitrogen is added to the molten steel before teeming to impart strain aging properties thereto. In the hot rolled or cold rolled condition the steels have a yield strength of 50 to ksi, which is increased to a range of 70 to ksi after straining and aging.

U.S. Pat. No. 3,673,008, issued June 27, 1972, to M. E. Wood, discloses carbonitriding of a columbiumcontaining steel by heating an article formed from the cold worked steel to a temperature above the strain recrystallization temperature but below the A critical temperature of the steel in a carbonitriding atmosphere containing hydrocarbons and ammonia.

The purpose of the Wood patent is to prevent strain induced grain coarsening by addition of from 0.006 percent to 0.018 percent columbium to a carbon steel containing from 0.05 percent to 0.15 percent carbon. The carbonitriding is stated to produce a hard, wearresistant case on the formed article. A person skilled in the art would conclude that the undesirable ferritic grain coarsening is inhibited by columbium-carbide precipitates. Such carbide precipitates must exist in a fine dispersion in order to provide resistance to grain coarsening. A fine dispersion is obtained in steels of the type disclosed in Wood because of the high carbon content which significantly lowers the A, critical tem' perature (to about 1,333F). The behavior of low carbon steels to which the present invention relates would be considered non-analogous to that of medium or high carbon steels since low carbon steels have a substantially higher A, critical temperature, and columbiumcarbide particles formed by following the process of the Wood patent would be expected by a person skilled in the art to be too coarse to inhibit ferritic grain growth.

The case hardening of relatively massive parts by nitriding, as practiced conventionally, is distinguishable from the concept of strengthening hot rolled or cold rolled low carbon sheet stock. The prior art suggestions of addition of alloying elements such as columbium for the purpose of case hardening or prevention of grain coarsening, would not provide a person skilled in the art with a teaching which would lead to the solution of the problem of increasing the strength of stamped or deep drawn parts formed from deep drawing quality steel sheet stock.

Despite the above background, no successful approach has as yet been made to the problem of increasing the strength of deep drawn parts or stampings formed from sheet stock without loss of the necessary ductility and drawability of the steel required to make the part. Present practice is still governed by the fundamental precept that enhancement of strength is accomplished only by a sacrifice in ductility, drawability, and- /or stretchability. To the best of applicants knowledge the prior art has never previously suggested the application of alloy-nitrogen precipitation strengthening to a deep drawing quality, low carbon steel. As is well known, when such steel in sheet form is subjected to drawing or stamping, the finished article will have areas of low yield strength where the part has not been work hardened by straining or deformation, and will have other areas of high yield strength hardened by straining or deformation in forming the article. Typically the yield strength of unstrained areas is the same as or slightly higher than the yield strength of the steel sheet from which the part was formed, i.e., about 20 30 ksi. The areas which have been work hardened may have yield strengths ranging upwardly from about 30 ksi to about 80 or 100 ksi, depending upon the severity of straining or deformation. When such article is subjected to heat treatment, the strained areas exhibit recrystallization and excessive grain growth, with consequent undesirable softening.

The prior art approach, illustrated by the abovementioned Bucher patent, which utilizes strain-aging by carbon or nitrogen to strengthen a formed article, cannot be applied where deep drawing properties are required. Steels which can be strengthened more than a negligible amount by strain-age hardening inherently possess relatively high strength and low ductility in the hot rolled or cold rolled condition and hence cannot be subjected to deep drawing. Moreover, the gain in strength resulting from strain-age hardening is relatively small, on the order of about 10 ksi, and virtually no strengthening in unstrained areas of parts formed from such steels can be achieved.

SUMMARY It is a principal object of the present invention to provide a process for producing articles by drawing or stamping from a deep drawing quality steel of a specific composition and subsequently to treat the articles after forming by a nitriding treatment which enhances the strength thereof.

It is a further object of the invention to provide a cold rolled sheet stock, and a method for production thereof, in the thickness range of 0.02 to 0.09 inch having a yield strength of at least about 70 ksi.

The present invention provides a method of increasing the yield strength of a low carbon steel sheet stock of deep drawing quality, and articles formed therefrom, by adding to a killed, drawing quality steel a nitrideforming alloying element chosen from the group consisting of titanium, columbium, zirconium, and mixtures thereof, in amounts such that titanium in solution at room temperature is from about 0.02 percent to 0.2 percent, columbium in solution is from about 0.025 percent to 0.3 percent, and zirconium in solution is from about 0.025 percent to 0.3 percent by weight, reducing the steel to final thickness, annealing if an article is to be formed therefrom, and heating the steel, or articles formed therefrom, in an atmosphere comprising ammonia and hydrogen at a temperature between l,l and l,350F for a period of time sufficient to cause reaction of the nitride-forming element with the nitrogen of the ammonia to form small, uniformly dispersed nitrides. The concentration of ammonia in the annealing atmosphere ranges between about 2 percent and percent by volume and must be insufficient, at the temperature and time involved, to permit formation of iron nitride or an iron nitrogen austenite.

Unlike prior art nitriding practice, the alloy-nitrogen precipitation strengthening process of the present invention avoids the formation of an iron-nitrogen austenitic structure by heating at a higher temperature, for a shorter time, and with a lower ammonia concentration in the atmosphere than a typical nitriding operation.

Moreover, no quench is applied after the heat treatment, contrary to conventional practice in nitriding. The present invention does not obtain or seek the properties desired in nitriding other types of steels, viz. high surface hardness at elevated temperature, great wear resistance, increased endurance limit, and resistance to certain types of corrosion.

The process of the present invention involves relatively low and hence inexpensive alloying additions to low carbon steel, and relatively low heat treatment temperature for a relatively short period of time, thereby providing a commercially economical process which does not require specialized facilities or equipment.

A composition suitable for the practice of the invention comprises, in broad ranges:

Carbon About 0.002 to 0.015% Nitrogen Up to about 0.012 Aluminum Up to about 0.08 Manganese About 0.05 to 0.6

ulfur Up to about 0.035

Oxygen Up to about 0.0]

Phosphorus Up to about 0.01

Silicon Up to about 0.015

Titanium About 0.02 to 0.2 in solution Columbium About 0.025 to 0.3 in solution Zirconium About 0.025 to 0.3 in solution Iron Remainder, except for incidental impurities In the above composition all percentages are by weight, and the titanium, columbium and zirconium may be present singly or in admixture, the sum total not exceeding about 0.3 percent.

DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred composition is as follows:

impurities As indicated previously, the ammonia concentration in the annealing atmosphere is maintained at a concentration sufficiently low, at the temperature and time involved, to avoid the formation of iron nitride or an austenitic structure, thereby avoiding high surface hardness, low toughness and embrittlement. Preferably, the atmosphere in which the heat treatment is conducted contains from 3 percent to 6 percent ammonia by volume, with the balance hydrogen. An inert gas, such as nitrogen or argon may be substituted for part of the hydrogen, provided proper adjustments are made in the ratio of ammonia to hydrogen contents so that the formation of iron nitride or an iron-nitrogen austenite does not occur.

It has been found that a heat treatment conducted in this atmosphere within the temperature range of l,l00 to 1,350F, preferably l,l00 to 1,300F, results in relatively rapid diffusion of nitrogen into the steel and reaction of the nitrogen with the nitride-forming alloying element to form small, uniformly dispersed nitride particles, probably ranging in size between about 20 and about 30 Angstroms. A time of l to 3 hours at temperature is ordinarily sufficient.

When heat treating a drawn or stamped article having strained and work hardened areas, it is preferred to provide both titanium and columbium, or both titanium and zirconium, as the nitride-forming elements. The presence of at least about 0.025 percent columbium or zirconium (as determined by analysis at room temperature) prevents the recrystallization and consequent softening of the strained areas of the formed article when subjected to heat. Thus, in the preferred practice of the invention as applied to deep drawn or stamped articles, the yield strength in the unstrained areas is increased to a minimum of 50 ksi and the yield strength of the strained areas is maintained or even increased.

Where the method of the invention is applied to the strengthening of a cold rolled sheet stock in the unformed condition having a thickness ranging between about 0.02 inch and about 0.09 inch, preferably 0.02 to 0.06 inch, the yield strength will be increased to at least about 70 ksi, a value never previously attainable in a low carbon steel. Such a product has sufficient formability to permit fabrication into articles of various types (other than deep drawn) .wherein bends are mainly involved.

According to studies reported by L. S. Darken and R. W. Gurry in Physical Chemistry of Metals, McGraw- Hill Book Company, Inc. (1953), pages 372 395, the

maximum ammonia concentrations which can be used within the temperature ranges of l,l00 to 1,350F and still avoid the formation of iron nitride, are as follows:

ll00F about ammonia l200F about 6% ammonia l300F about 3% ammonia 1350F about 2% ammonia l,l00F the rate of diffusion is so slow that the time required at temperature is commercially uneconomical. Above 1,350F, the ammonia concentration must be kept so low that the driving force for diffusion becomes insufficient. In addition, the nitrides formed at l,350F and above are coarser in size and hence contribute less strengthening effect. Finally, when heat treating deep drawn or stamped articles having cold worked areas, a temperature above 1,350F should be avoided because of excessive grain growth and consequent softening.

Within a preferred temperature range of 1,l00 to l,300F and a preferred ammonia concentration of 3 percent to 6 percent by volume, the heat treatment time can range between about 1 hour and about 2 hours. Under such conditions nitrogen diffuses to a depth sufficient to increase the average yield strength of material having an as-received yield strength of 30 ksi to a minimum of ksi.

The thickness of steel sheet treated in accordance with the process of the invention does not constitute a limitation, although its greatest utility resides in the treatment of hot rolled thin bar ranging in thickness from about 0.06 inch to 0.25 inch and cold rolled strip material ranging in thickness from about 0.02 to about 0.09 inch. Thin cold rolled material (i.e., up to about 0.06 inch) heat treated at about l,300F for l to 2 hours will be strengthened by alloy-nitrogen precipitation in finely dispersed form substantially all the way through the thickness and will achieve a yield strength of about 70 ksi. Thicker hot rolled material can be heat treated at somewhat lower temperatures, in which case it will be nitridedonly part way through, but to a depth sufficient to obtain an average yield strength in excess of 50 ksi and up to about 85 ksi.

Experimental data are presented in the tables below for a series of heats of steels containing titanium, columbium, zirconium or mixtures thereof. For purposes of comparison a typical drawing quality aluminumkilled steel sheet containing no nitride-forming alloy 40 other than aluminum has been included.

TABLE I Composition Percent by Weight Example Hcnt C N O S Mn Al Ti Ch Zr B l090073 .040 .0 I 5 .0029 .30 .()6X( tull) A .()35(in sol) Z Vlt-lS-Z .0042 .0036 .00Jl .019 .3] .004 .109

3 V8454 .004] .0045 .0022 .019 .3l .031 .1 l0 4 X00l(\l\' ,0044 .0057 .00l2 .01] .4 .030 .lZ 5 2250350-V .002 .0036 .0l9 .32 .047 .095 .066 h 126077X-Y .004] .003l .01] .33 .029 .049 .039 7 V7063 .0055 .0050 .00 l X .017 .30 .lZ .l) K ZZhUfifib-Y .004 .0030 .015 .33 .040 .084 .063 9 Z2609l4\' .003 .0045 .014 .32 .044 .078 .058 M The above values represent equilibrium between ammonia-hydrogen mixtures and solid phases of the iron nitride system at one atmosphere pressure.

It will be apparent from the above information that the temperature and ammonia concentration are interdependent and should be varied inversely with respect to one another in the practice of the present process. Similarly, time is a further interdependent variable also inversely proportional to the temperature and ammonia concentration. It has been found that the rate of diffusion of the nitrogen into the steel is the controlling factor since the reaction rate of nitrogen with the alloying elements is relatively rapid. Below a temperature of 7 8 annealed. Samples were also subjected to 20 percent in order to simulate strained and/or deformed areas of cold reduction to 0.032 inch thickness after annealing drawn articles.

TABLE 11 Heat 'l rcnrcd in 37! NH 97% H By Volume Exnniple I Y.S T.S. '71 Condition ksl ksi Elong. Y.P.E. "/[N As-Rcceivcd 26.6 43.9 46.0 0.0 .015 1100F-1 hr. 35.9 50.5 37.0 3.7 .015

- -2 hrs. 38.8 53.5 30.5 3.3 1200F-1 hr. 47.9 62.4 26.5 3.0 .056 -2, hrs.. 52.0 m 64.9 23.0 3.0 .078 1300F1 hr. 51.4 70.5 24.5 2.2 .13

-2 hrs. 51.4 71.2 21.5 1.6

Colled Rolled 20% 59.6 59.6 10.5 1100F-1 hr. 51.7 63.0 19.0 2.6 2 hrs. 49.9 62.0 25.5 2.5 1200F1 hr. 56.1 69.0 19.0 2.6 -2 hrs. 57.1 69.9 19.0 2.6 1300F-1 hr. 48.9 70.9 20.0 0.0 k -2 hrs. 48.9 V 73.7 17.0 0.0

Example 4 Y.S. T5. "/1 71 Condition ksi ksi Elong. Y.P.E. "/1 N As-Received 23.8 48.6 37.0 0.0 0057 1100F-1 hr. 39.8 50.8 35.5 5.0 .012

-2 hrs. 55.9 65.1 15.0* 4.3 1200F-1 hr. 51.2 59.3 18.0 2.7 .029 -2 hrs. 71.2 79.9 12.0 3.3 .069 1300F-1 hr. 56.6 70.9 20.0 3.2 .074

-2 hrs. 62.5 77.1 22.0 2.9 n

Cold-Rolled 20% 67.9 73.2 4.0 0.0 .0057

1100F-1 hr. 60.8 66.8 14.0 1.8

-2 hrs. 74.6 78.1 9.0 0.8 1200F-1 hr. 66.7 71.0 13.0 4.3 -2 hrs. 82.3 86.9 11.0 3.9 1300F-1 hr. 80.8 87.3 13.0 3 5 -2 hrs. 78.9 85.6 50* l 8 4 Example 5 Y.S. T5. '71 "/1 Condition ksi ksi Elong. Y.P.E. /1 N As Rcccived 22.1 45.5 40.0 0.0 .0036 1l00F-1 hr. 37.4 55.6 28.0 0.3 .010

-2 hrs. 66.3 76.7 19.0 1.1 1200F-1 hr. 70.0 211.1 17.0 0.9 .029 -2 111's. 103.6 111.6 11.0 2.0 .079 1300F-l hr. 87.0 95.5 15.5 1.9 .098

-2 hrs. 89.4 100.4 13.0 1.5

Cold-Rolled 20% 69.0 73.4 3.5 0.0 .0036

1100E-1 hr. 65.6 70.3 11.0 0.0 -2 hrs. 90.5 92.9 9.0 0.0 w l200F-1 hr. 90.5 94.5 11.5 1.7 -2 hrs. 114.3 118.4 10.0 2.6 w 1300F-1 hr. 94.8 100.1 12.5 3.5 '2 hrs. 97.2 103.5 12.0 2.5

Example 6 Y.S. '1.S. 71 Condition ksi ksi Elong. Y.P.E. ZN

As-Rcceivcd 20.7 44.8 40.5 0 .0031 1100F-l hr. 43.8 55.7 29.5 2.5 .0093

-2 hrs. 68.0 74.1 19.0 3.6 1200F-| hr. 74.8 82.1 15.0 2.7 .032 -2 hrs. 89.6 98.7 13.0 2.0 .067 1300F-1 hr. 73.8 81.9 17.0 2.5 .080

(old-Rolled 20'4 63.4 68.2 5.0 0.0 .0031

11001"-| 111'. 64.2 09.3 14.0 1.7 -2111.\'. 84.0 88.3 11.0 "17 |2001"-1 111. 82.8 86.0 11.0 3.0 4 -2 hrs. 94.3 98.7 14.0 2.8 1300F-1 hr. 142.4 88.6 16.0 2.11 -2 hrs. 81.4 88.6 16.0 2.8

TABLE II- Continued Heat Treated in 3% NH 97% ll. By Volume 'llrukc near or outside gage mark.

Table 11 indicates that the aluminum-killed drawing quality steel of sample 1 showed very little strengthening when nitrided under the same conditions as the remaining steels of examples 2 7. The moderate increase in yield strength is due primarily to the return of the yield point elongation. In addition, some strengthening occurs as a result of nitrogen in solid solution in the steel. V i

A more direct comparison of the strengthening effect of titanium to that of aluminum is obtained from examples 2 and 3, example 2 containing only titanium as a nitride former, with example 3 containing the same amount of titanium plus 0.031 percent aluminum. It is apparent that no beneficial effect with respect to strengthening is obtained by addition of aluminum. The only difference is that the steel of example 2 containing only titanium developed yield point elongation while that of example 3 did not do so at the same total nitrogen concentrations. This is of course due to the fact that aluminum was available to scavenge nitrogen, thus resulting in less nitrogen in solid solution.

It is further apparent that the increase in yield strength produced in the columbium-bearing and zirconium-bearing steels of examples 4 and 7, respectively, is not as great as that for the titanium-bearing steels. However, yield strengths in excess of 50 ksi were obtained in both cases by heat treatment at l,200 F.f 1 h ur Example 5 was an embodiment of a relatively highly alloyed titanium and columbium-bearing steel which exhibited an increase in yield strength substantially the same as that of examples 2 and 3. H v

Example 6, illustrative of lower alloying additions of titanium and columbium than example 5, exhibited significant increase in yield strength, although not as high as that of the more highly alloyed example M V The yield strengths reported for samples subjected to cold reduction (simulating the strain or deformation resulting from deep drawing) show that the en g, wh h. aqa m an fih ntsq tati npf alloy nitrides is additive to the cold work strengthening so that a net gain in strength is obtained even though there is a small loss in strength due to partial recovery. The strength advantage in nitrided cold worked material over nitrided as-annealed material is attributable to alloy nitride nucleation and precipitation on dislocations and to enhanced solubility of nitrogen in a strained lattice.

The elongation values after nitriding are relatively high in view of the yield strength levels attained. These elongation values indicate that some limited forming could be performed after nitriding strip material such as bending or a restrike operation. The samples subjected to 20 percent cold reduction increased in elongation values along with an increase in yield strength, because of recovery.

As indicated previously the heat treatment step of the present process results in an increase in nitrogen in solid solution in the steel as well as nitrogen combined as nitrides with titanium, columbium, and/or zirconium. It has been found that the total amount of nitrogen taken up by the steel can exceed that required to satisfy normal equilibrium solution requirements plus that needed to convert the alloys to nitrides. This excess nitrogen can be attributed to nitrogen trapped on dislocations, adsorbed at the nitride-ferrite interface, and as enhanced lattice solubility in strained ferrite.

Of further significance in Table 11 are the values reported for nitriding at 1,300F in 3 percent ammonia for 2 hours. In most instances a decrease in yield strength from the maximum values obtained at l,200F for two hours occurred, due to the formation of coarser alloy nitride particles. A thin austenite rim formed on the surfaces of samples nitrided at 1,300F; therefore, to avoid the formation of an iron nitrogen austenite ,rim, the ammonia concentration should be slightly less than 3 percent at l,300F for a 2 hour heat treatment.

A comparison of the strengthening achieved by heat treating in a 3 percent ammonia 97 percent hydrogen mixture with that achieved in a 6 percent ammonia 94 percent hydrogen mixture is given in Table 11] below.

TABLE 111 Effect of Ammonia Concentration Cold-Rolled and Annealed 0.40" Sheets 3%NH 97%H 6%NH 94%H Example & Y.S. T.S. Y.s. T.S. Aust. Condition ksi ksi %Elong. %Y.P.E. %N ksi ksi %Elong. %Y.P.E. Rim

Steel 2 1100F-1 hr. 37.6 54.2 23.5 0.7 .012 57.3 71.4 18.0 0.8 No

1200F-1 hr. 76.4 92.6 14.0 1.4 .047 109.7 122.3 12.5 0.5 No

1300F-1 hr. 88.3 98.3 14.0 1.5 .094 93.0 104.4 3.0 0.7 Yes Steel 3 1100 F-1 hr. 33.5 51.2 30.0 0.7 .011 52.8 67.5 20.0 0.5 No

1200F-l hr. 79.9 89.5 11.0 0.0 .047 121.3 128.6 8.0 0.0 No

1300F-1 hr. 88.1 98.6 12.5 0.0 .11 98.3 104.7 1.5 0.0 Yes Steel 4 1l00F-1 hr. 39.8 50.8 35.5 5.0 .012 44.8 55.7 28.0 5.0 No

1200F-1 hr. 51.2 59.3 18.0 2.7 .029 73.6 83.4 9.5 2.5 No

1300F-1 hr. 56.6 70.9 20.0 3.2 .074 64.9 79.7 15.0 2.9 Yes Steel 5 1100 F-1 hr. 37.4 55.6 28.0 0.3 .010 54.8 68.8 18.0 0.7 No

1200F-1 hr. 70.0 81.1 17.0 0.9 .029 107.0 114.5 5.5 0.0 No

, 1300F-1 hr. 87.0 95.5 15.5 1.9 .098 97.9 110.0 4.0 1.2 Yes Steel 6 1100F-1 hr. 43.8 55.7 29.5 2.5 .0093 56.4 66.1 18.5 3.8 No

1200F-1 hr. 74.8 82.1 15.0 2.7 .032 98.2 106.7 12.0 3.5 No

1300F-1 hr. 73.6 81.9 17.0 2.5 .080 82.6 96.7 11.0 1.7 Yes Steel 7 1100F-1 hr. 32.7 49.8 37.5 1.5 40.6 54.7 30.0 3.9 No

1200F-1 hr. 54.1 62.7 24.0 5.1 66.7 76.1 19.0 4.0 No

1300F-1 hr. 62.9 73.6 17.5 3.3 72.9 84.4 14.0 2.5 Yes It is apparent from Table 111 that any given steel achieves a higher yield strength when nitrided in 6 percent ammonia under the same time and temperature conditions than is attained by nitriding in 3 percent ammonia. Higher strength is obtained by nitriding at 1,200F for 1 hour in 6 percent ammonia than by nitriding at 1,200F for 2 hours in 3 percent ammonia. However, due to diffusion phenomena, the surface to mid-thickness strength gradient would be greater in a 6 percent ammonia atmosphere than in a 3 percent ammonia atmosphere. For some applications it may be desirable to obtain a lower average strength with a lesser gradient to mid-thickness. The present invention makes it possible to select readily temperature, time and ammonia concentrations which'will result in a wide range of average yield strengths and surface to mid-thickness strength gradients.

Table 111 again indicates that nitriding in 6 percent ammonia at 1,300F results in formation of an iron- I nitrogen austenite rim which will transform either to martensite or an eutectoid structure, depending upon the cooling rate. In example 5, an austenite rim about 1 mil thick resulted from annealing in 6 percent ammo nia at 1,300F for 1 hour.

As the thickness of the steel stock subjected to the heat treatment of the presentinvention increases, the

time required to reach saturation at the equilibrium nitrogen content in solution (for a given temperature and ammonia concentration in the atmosphere) increases as the square of the thickness. For example, for nitrogen diffusion in pure iron at l,200F, to reach an average fractional saturation (i.e., N Avg/N Equil.) of 0.7 it has been found that 1 hour is required for sheet stock of 0.040 inch thickness while 5.6 hours is required for sheet stock of 0.090 inch thickness. However, an important feature of the present invention is the discovery that marked increases in average yield strength can be realized within relatively short times, (i.e., not more than 2 hours), by partial alloy-nitrogen precipitation strengthening. Table IV below indicates the substantial increase in yield strength achieved by nitriding the titaniumcoluumbium bearing steel of example 8 in a 3 percent ammonia 97 percent hydrogen atmosphere within the temperature range of l,l00 1,300F for 1 2 hours. It will be noted that nitriding at 1,200F for only one hour resulted in an average yield strength of 66.5 ksi. Even greater strengthening could be attained by heat treating for longer periods of time or by increasing the ammonia concentration to 6 percent. In Table IV 1,300F again proved to be an unacceptable temperature when using a 3 percent ammonia atmosphere because of formation of an austenite rim.

Properties of 0.090 lnch Hot Rolled Steel of Exam 1e 8 Heat Treated in 3% NH 97% 2 The criticality of providing at least about 0.02 percent titaniurn in solution is illustrated in Table V below.

TABLE V concentrations, just less than that which results in the formation of iron nitrides or an iron-nitrogen austenite,

Effect of Amount of Available Nitride-Forming Element In Table V a steel of the invention containing 0.077 0 percent total titanium, 0.037 percent total columbium, 0.31 percent aluminum, 0.0035 percent nitrogen, and remainder substantially iron, was carburized from an original carbon level of 0.0044 percent to a carbon level of 0.010 percent and to saturation with carbon in order to vary the amount of titanium in solution available to react with ammonia in the nitriding operation. It is apparent from Table V that a substantial decrease in yield strength occurs progressively with decrease of available titanium in solution from 0.047 percent to 0.025 percent and to 0 percent successively.

In order to ascertain the degree of strengthening contributed by nitrogen taken into solid solution in the steel, the samples of Table V were denitrided by heating in a hydrogen atmosphere at about 1,200F for 2 hours. Table V reports the yield strength in the deniis desirable from the standpoint of producing maximum strengthening in the shortest possible time. However, it has been discovered that if nitriding conducted for the purpose of strengthening (and which results in an unde sirably high excess nitrogen content in solid solution) is followed by a denitriding step, such as annealing in hydrogen gas, the excess nitrogen is removed with only a 10 to 20 percent reduction in yield strength. Removal of the excess nitrogen eliminates welding porosity and significantly reduces the ductile to brittle transition temperature while improving the impact energy values. The present invention thus provides low carbon, high strength steel stock suitable for welding applications.

Table VI demonstrates the above observations regarding the effect of denitriding. A steel of the invention initially containing 0.006 percent carbon, 0.077 percent titanium, 0.037 percent columbium, 0.03 l perity resulting from the liberation of this excess nitrogen as nitrogen gas. These problems can be overcome by special welding techniques. The use of high ammonia trided condition and further sets forth the differential Cent um n Percent g n. and balance at each of the different carbon contents, from which it n ially ron, ccldrolled to 0.058 inch thickness is apparent that nitrogen taken into solid solution conand annealed, Was nltf'ldad a5 lndlcated In Table tributes about 20 ksi to the yield strength. It is further Samp e A was o denitrided, a p B a pa t y apparent that denitrided material at the 0.010 percent e rlded, and S mple C was St rthe f en dcarbon level (resulting in 0.025 percent available tita- 40 Both the yield strength and the ductile to brittle transinium in solution) retains a substantially increased yield tion temperature decreased gradually with the decrease strength of 65.9 ksi in the denitrided condition. H in nitrogen in solution.

TABLE VI i Effect of Denitriding Ductile to Y.S. T.S. Measured Calc. Brittle Transi- Sample Treatment ksi ksi %Elong. %Ntotal %N(soln.) tion Temp.F

A Nitrided in 6%NH,-94%H, 88.3 100.7 l8 0.10 0.067 0 l200F 3 hrs.

B Nitrided as in A, then 80.8 92.0 l7 0.053 0.020

denitrided in H2 l200F 2 hrs.

C Nitrided as in A, then 77.l 87.] 16 0.043 0.010 60 denitrided l200F 4 hrs.

It has been discovered that the excess nitrogen con- 60 Steel having the composition specified herein can be tent of the steels following alloy-nitrogen precipitation melted by any conventional operation such as open strengthening at higher allowable ammonia contents hearth, basic oxygen furnace or electric furnace. The can present weldability problems and result in high molten steel is then vacuum degassed in order to ductile to brittle notched sheet Charpy impact transiachieve the desired carbon and nitrogen levels, killed tion temperature. The welding problems involve porospreferably with Al, and'the nitride-forming alloying element or elements are added to the ladle after degassing with suitable mixing. The melt is then teemed into ingots, or cast into slabs. The solidifed ingots or slabs are then subjected to conventional hot rolling and to conventional subsequent processing steps to obtain sheet stock of the desired final thickness. The steel is then subjected to the process of the present invention either in the form of sheet or strip, or after forming into articles by drawing or stamping.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

l. A method of increasing the yield strength of a low carbon steel sheet stock, which comprises:

providing a deep drawing quality steel containing from about 0.002 percent to about 0.015 percent carbon, up to about 0.012 percent nitrogen, up to about 0.08 percent alluminum, and a nitrideforming element chosen from the group consisting of titanium, columbium, zirconium, and mixtures thereof, in amounts such that titanium in solution is from about 0.02 to about 0.2 percent, columbium in solution is from about 0.025 percent to about 0.3 percent, and zirconium in solution is from about 0.025 percent to about 0.3 percent, all percentages being by weight;

reducing said steel to final thickness; and heating the resulting sheet stock in an atmosphere comprising ammonia and hydrogen at a temperature between l,l and 1,350F for a period of time sufficient to cause reaction of said nitride-forming element with the nitrogen of said ammonia to form small, uniformly dispersed nitrides, whereby to increase the average yield strength of said sheet stock to a minimum of 60 ksi, the concentration of ammonia in said atmosphere ranging between about 2 percent and percent by volume and being insufficient, at the temperature and time involved, to permit formation of iron nitride.

2. The method of claim 1, wherein said steel initially consists essentially of from about 0.002 percent to about 0.015 percent c'arbon, up to about 0.012 percent nitrogen, up to about 0.08 percent aluminum, from about 0.05 percent to about 0.6 percent manganese, from about 0.02 percent to about 0.2 percent titanium in solution, up to about 0.035 percent sulfur, up to about 0.01 percent oxygen, residual silicon and phosphorus, and remainder iron except for incidental impurities, all percentages being by weight.

3. The method of claim 1, wherein said steel initially consists essentially of from 0.002 percent to about 0.015 percent carbon, up to about 0.012 percent nitrogen, up to about 0.08 percent aluminum, from about 0.05 percent to about 0.6 percent manganese, from about 0.025 percent to about 0.3 percent columbium, up to about 0.035 percent sulfur, up to about 0.01 percent oxygen, residual silicon and phosphorus, and remainder iron except for incidental impurities, all percentages being by weight.

4. The method of claim 1, wherein said steel initially consists essentially of from about 0.002 percent to about 0.015 percent carbon, up to about 0.012 percent nitrogen, up to about 0.08 percent aluminum, from about 0.05 percent to about 0.6 percent manganese,

from about 0.025 percent to about 0.3 percent zirconium, up to about 0.035 percent sulfur, up to about 0.01 percent oxygen, residual silicon and phosphorus,

percentages being by weight.

5. The method of claim and remainder iron except for incidental'lmpurities, all

1, wherein. said steel initially.

consists essentially of less than about 0.010 percent carbon, from about 0.05 percent to about 0.6 percent manganese, from about 0.02 percent to about 0.04 percent total aluminum, up to about 0.004 percent nitrogen, up to about 0.035 percent sulfur, up to about 0.01 percent oxygen, residual silicon and phosphorus, from about 0.08 percent to about 0.10 percent total titanium, from about 0.03 percent to about 0.06 percent total columbium, and remainder iron except for incidental impurities, all percentages being by weight.

6. The method of claim 1, wherein said steel initially consists essentially of less than about 0.010 percent carbon, from about 0.05 percent to about 0.6 percent manganese, from about 0.02 percent to about 0.04 percent total aluminum, up to about 0.004 percent nitrogen, up to about 0.035 percent sulfur, up to about 0.01 percent oxygen, residual silicon and phosphorus, from about 0.08 percent to about 0.10 percent total titanium, from about 0.03 percent to about 0.06 percent total zirconium, and remainder iron except for incidental impurities, all percentages being by weight.

7. The method of claim 1, wherein said sheet stock is cold rolled to a thickness of from about 0.02 inch to 0.09 inch, and wherein said heating step increases the yield strength of said sheet stock to at least about ksi.

8. The method of claim -I, wherein said atmosphere contains from 3 percent to 6 percent ammonia by volume, and remainder hydrogen.

9. The method of claim 1, wherein said atmosphere contains ammonia and hydrogen, and the balance an inert gas, with the ammonia to hydrogen ratio adjusted in such manner that the formation of iron nitride or iron-nitrogen austenite is avoided.

10. The method of claim 7, wherein said heating is conducted at 1,100 to 1,300F for a period of time inversely proportional to the temperature and directly proportional to the square of the thickness.

11. A method of increasing the yield strength of an article formed from a low carbon steel sheet stock of deep drawing quality, which comprises:

providing a steel containing from about 0.002 percent to 0.015 percent carbon, up to about 0.012 percent nitrogen, up to about 0.08 percent aluminum, and a nitride-forming element chosen from the group consisting of titanium, columbium, zirconium, and mixtures thereof, in amounts such that titanium in solution is from about 0.02 percent to 0.2 percent, columbium in solution is from about 0.025 percent to 0.3 percent, and zirconium in solution is from about 0.025 percent to 0.3 percent, all percentages being by weight;

reducing said steel to final thickness;

annealing to soften and impart excellent drawing quality properties;

forming said article from said reduced and annealed steel; and

strengthening said article in an atmopshere comprising ammonia and hydrogen at a temperature between 1, and 1,350F for a period of time sufficient to cause reaction of said nitride-forming element with the nitrogen of said ammonia, whereby to-increase the average yield strength of said article to a minimum of 50 ksi, the concentration of ammonia in said atmosphere being between about 2 perce-nt-and about 10 percent by volume and being insufficient, at the temperature and time involved,

to permit formation of iron nitride or iron-nitrogen austenite.

12. The method of claim 11, wherein said steel initially consists essentially of from about 0.002 percent to about 0.015 percent carbon, up to about 0.012 percent nitrogen, up to about 0.08 percent aluminum, from about 0.05 percent to about 0.6 percent manganese, from about 0.02 percent to about 0.2 percent titanium in solution, up to about 0.035 percent sulfur, up to about 0.01 percent oxygen, residual silicon and phosphorus, and remainder iron except for incidental impurities, and percentages being by weight.

13. The method of claim 11, wherein said steel initially consists essentially of from about 0.002 percent to about 0.015 percent carbon, up to about 0.012 peri i 0.01 percent oxygen, residual silicon and phosphorus,

cent nitrogen, up to about 0.08 percent aluminum,

from about 0.05 percent to about 0.6 percent manganese, from about 0.025 percent to about 0.3 percent columbium in solution, up to about 0.035 percent sulfur, up to about 0.01 percent oxygen, residual silicon and phosphorus, and remainder iron except for incidental impurities, and percentages being by weight.

14. The method of claim 11, wherein said steel initially consists essentially of from about 0.002 percent to about 0.015 percent carbon, up to about 0.012 percent nitrogen, up to about 0.08 percent aluminum,

from about 0.05 percent to about 0.6 percent managense, from about 0.025 percent to about 0.3 percent zirconium in solution, up to about 0.035 percent sulfur, up to about 0.01 percent oxygen, residual silicon and phosphorus, and remainder iron except for incidental impurities, and percentages being by weight.

15. The method of claim 11, wherein said steel initially consists essentially of less than about 0.010 percent carbon, from about 0.05 percent to about 0.6 percent manganese, from about 0.02 percent to about 0.04 percent total aluminum, up to about 0.004 percent nitrogen, up to about 0.035 percent sulfur, up to about from about 0.08 percent to about 0.10 percent total titanium, from about 0.03 percent to about 0.06 percent total columbium, and remainder iron except for incidental impurities, all percentages being by weight.

16. The method of claim 11, wherein said steel initially consists essentially of less than about 0.010 percent carbon, from about 0.05 percent to about 0.6 percent manganese, from about 0.02 percent to about 0.04 percent total aluminum, up to about 0.004 percent nitrogen, up to about 0.035 percent sulfur, up to 0.0l

percent oxygen, residual silicon and phosphorus, from about 0.08 percent to about 0.10 percent total titanium, from about 0.03 percent to about 0.06 percent total zirconium, and remainder iron except for incidental impurities, all percentages being by weight.

17. The method of claim 11, wherein said atmosphere contains from 3 percent to 6 percent ammonia by volume, and remainder hydrogen.

18. The method of claim 11, wherein said atmosphere contains ammonia and hydrogen, and the balance an inert gas, with the ammonia to hydrogen ratio adjusted in such manner that the formation of iron nitride or iron-nitrogen austenite is avoided.

19. The method of claim 11, wherein said heating is conducted at 1,100 to 1,300F for a period of time inversely proportional to the temperature, and directly proportional to the square of the thickness.

20. The method of claim 11, wherein said article is deep drawn.

21. The method of claim 1, including as a final step denitriding said sheet stock in a hydrogen atmosphere at a temperature between about l,100 and l,350F for a period of time sufficient to decrease the ductile to brittle transition temperature to at least about 60F.

Page 1 of 2 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Q Patent No. 3,847,682 Dated November 12, 1974 Inventor(s) Rollin E. Hook It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 8 (Table II), between Example 1 and Example 4 there should be inserted the following:

Examgle 2 Y.S. T.S.

' Condition ksi ksi ZElong %Y P E N As-Received 18.3 42.1 42.0 0.0 .0036

1100F-1 hr. 37.6 54.2 23.5 0.7 .012

a 2 hr. 71.0 81.5 17.0 1.4

1200F1 hr. 76.4 92.6 14.0 1.4 .047

-2 hrs. 102.6 114.5 12.5 1.5 .079

l300F1 hr. 88.3 98.3 14.0 1.5 .094

t -2 hrs. 85.2 97.6 13.0 0.0

Cold-Rolled 20% 60.0 63.1 7.0 0.0 .0036

1100F -1 hr. 57.9 64.3 16.0 0.0

a 2 hrs. 85.0 91.2 12.0 0.8

1200F-l hr. 96.3 102.5 10.0 1.1

2 hrs. 107.0 113.5 12.0 1.8

1300F1 hr. 90.6 98.3 11.0 1 8 Q 2 hrs. 85.8 100.2 7.0* 0 0 CERTIFICATE OF CORRECTION Patent No.

D t Novelber 12, 1974 Inventor(s) Rollin E. Hook It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Condition As-Received ll00F-l hr.

2 hrs.

1200Fl hr.

2 hrs.

l300F-l hr.

2 hrs.

Cold-Rolled 207,

llOOF-l hr.

l200F-l hr.

2 hrs.

l300F-l hr.

2 hrs.

Example 3 Y.S. T .S.

E E ZElong. %Y.P.E. H

Column 13, line 14, "65.9" should read 8l.8--; "44.7" should read --65.9--; in the blank space (under the heading "C Saturated there should be inserted +4.7-.

[SEAL] Signed and Sealed this Seventh Day 0fv September 1976 Arrest:

RUTH C. MASON A nesting Officer 

1. A METHOD OF INCREASING THE YIELD STRENGTH OF A LOW CARBON STEEL SHEET STOCK, WHICH COMPRISES: PROVIDING A DEEP DRAWING QUALITY STEEL CONTAINING FROM ABOUT 0.002 PERCENT TO ABOUT 0.015 PERCENT CARBON, UP TO ABOUT 0.012 PERCENT NITROGEN, UP TO ABOUT 0.08 PERCENT ALLUMINUM, AND A NITRIDE-FORMING ELEMENT CHOSEN FROM THE GROUP CONSISTING OF TITANIUM, COLUMBIUM, ZIRCONIUM, AND MIXTURES THEREOF, IN AMOUNTS SUCH THAT TITANIUM IN SOLUTION IS FROM ABOUT 0.02 TO ABOUT 0.2 PERCENT, COLUMBIUM IN SOLUTION IS FROM ABOUT 0.025 PERCENT TO ABOUT 0.3 PERCENT, AND ZIRCONIUM IN SOLUTION IS FROM ABOUT 0.025 PERCENT TO ABOUT 0.3 PERCENT, ALL PERCENTAGES BEING BY WEIGHT; REDUCING SAID STEEL TO FINAL THICKNESS; AND HEATING THE RESULTING SHEET STOCK IN AN ATMOSPHERE COMPRISING AMMONIA AND HYDROGEN AT A TEMPERATURE BETWEEN 1,100* AND 1,350*F FOR A PERIOD OF TIME SUFFICIENT TO CAUSE REACTION OF SAID NITRIDE-FORMING ELEMENT WITH THE NITROGEN OF SAID AMMONIA TO FORM SMALL, UNIFORMLY DISPERSED NITRIDES, WHEREBY TO INCREASE THE AVERAGE YIELD STRENGTH OF SAID SHEET STOCK TO A MINIMUM OF 60 KSI, THE CONCENTRATION OF AMMONIA IN SAID ATMOSPHERES RANGING BETWEEN ABOUT 2 PERCENT AND 10 PERCENT BY VOLUME AND BEING INSUFFICIENT, AT THE TEMPERATURE AND TIME INVOLVED, TO PERMIT FORMATION OF IRON NITRIDE.
 2. The method of claim 1, wherein said steel initially consists essentially of from about 0.002 percent to about 0.015 percent carbon, up to about 0.012 percent nitrogen, up to about 0.08 percent aluminum, from about 0.05 percent to about 0.6 percent manganese, from about 0.02 percent to about 0.2 percent titanium in solution, up to about 0.035 percent sulfur, up to about 0.01 percent oxygen, residual silicon and phosphorus, and remainder iron except for incidental impurities, all percentages being by weight.
 3. The method of claim 1, wherein said steel initially consists essentially of from 0.002 percent to about 0.015 percent carbon, up to about 0.012 percent nitrogen, up to about 0.08 percent aluminum, from about 0.05 percent to about 0.6 percent manganese, from about 0.025 percent to about 0.3 percent columbium, up to about 0.035 percent sulfur, up to about 0.01 percent oxygen, residual silicon and phosphorus, and remainder iron except for incidental impurities, all percentages being by weight.
 4. The method of claim 1, wherein said steel initially consists essentially of from about 0.002 percent to about 0.015 percent carbon, up to about 0.012 percent nitrogen, up to about 0.08 percent aluminum, from about 0.05 percent to about 0.6 percent manganese, from about 0.025 percent to about 0.3 percent zirconium, up to about 0.035 percent sulfur, up to about 0.01 percent oxygen, residual silicon and phosphorus, and remainder iron except for incidental impurities, all percentages being by weight.
 5. The method of claim 1, wherein said steel initially consists essentially of less than about 0.010 percent carbon, from about 0.05 percent to about 0.6 percent manganese, from about 0.02 percent to about 0.04 percent total aluminum, up to about 0.004 percent nitrogen, up to about 0.035 percent sulfur, up to about 0.01 percent oxygen, residual silicon and phosphorus, from about 0.08 percent to about 0.10 percent total titanium, from about 0.03 percent to about 0.06 percent total columbium, and remainder iron except for incidental impurities, all percentages being by weight.
 6. The method of claim 1, wherein said steel initially consists essentially of less than about 0.010 percent carbon, from about 0.05 percent to about 0.6 percent manganese, from about 0.02 percent to about 0.04 percent total aluminum, up to about 0.004 percent nitrogen, up to about 0.035 percent sulfur, up to about 0.01 percent oxygen, residual silicon and phosphorus, from about 0.08 percent to about 0.10 percent total titAnium, from about 0.03 percent to about 0.06 percent total zirconium, and remainder iron except for incidental impurities, all percentages being by weight.
 7. The method of claim 1, wherein said sheet stock is cold rolled to a thickness of from about 0.02 inch to 0.09 inch, and wherein said heating step increases the yield strength of said sheet stock to at least about 70 ksi.
 8. The method of claim 1, wherein said atmosphere contains from 3 percent to 6 percent ammonia by volume, and remainder hydrogen.
 9. The method of claim 1, wherein said atmosphere contains ammonia and hydrogen, and the balance an inert gas, with the ammonia to hydrogen ratio adjusted in such manner that the formation of iron nitride or iron-nitrogen austenite is avoided.
 10. The method of claim 7, wherein said heating is conducted at 1,100* to 1,300*F for a period of time inversely proportional to the temperature and directly proportional to the square of the thickness.
 11. A method of increasing the yield strength of an article formed from a low carbon steel sheet stock of deep drawing quality, which comprises: providing a steel containing from about 0.002 percent to 0.015 percent carbon, up to about 0.012 percent nitrogen, up to about 0.08 percent aluminum, and a nitride-forming element chosen from the group consisting of titanium, columbium, zirconium, and mixtures thereof, in amounts such that titanium in solution is from about 0.02 percent to 0.2 percent, columbium in solution is from about 0.025 percent to 0.3 percent, and zirconium in solution is from about 0.025 percent to 0.3 percent, all percentages being by weight; reducing said steel to final thickness; annealing to soften and impart excellent drawing quality properties; forming said article from said reduced and annealed steel; and strengthening said article in an atmopshere comprising ammonia and hydrogen at a temperature between 1,100* and 1,350*F for a period of time sufficient to cause reaction of said nitride-forming element with the nitrogen of said ammonia, whereby to increase the average yield strength of said article to a minimum of 50 ksi, the concentration of ammonia in said atmosphere being between about 2 percent and about 10 percent by volume and being insufficient, at the temperature and time involved, to permit formation of iron nitride or iron-nitrogen austenite.
 12. The method of claim 11, wherein said steel initially consists essentially of from about 0.002 percent to about 0.015 percent carbon, up to about 0.012 percent nitrogen, up to about 0.08 percent aluminum, from about 0.05 percent to about 0.6 percent manganese, from about 0.02 percent to about 0.2 percent titanium in solution, up to about 0.035 percent sulfur, up to about 0.01 percent oxygen, residual silicon and phosphorus, and remainder iron except for incidental impurities, and percentages being by weight.
 13. The method of claim 11, wherein said steel initially consists essentially of from about 0.002 percent to about 0.015 percent carbon, up to about 0.012 percent nitrogen, up to about 0.08 percent aluminum, from about 0.05 percent to about 0.6 percent manganese, from about 0.025 percent to about 0.3 percent columbium in solution, up to about 0.035 percent sulfur, up to about 0.01 percent oxygen, residual silicon and phosphorus, and remainder iron except for incidental impurities, and percentages being by weight.
 14. The method of claim 11, wherein said steel initially consists essentially of from about 0.002 percent to about 0.015 percent carbon, up to about 0.012 percent nitrogen, up to about 0.08 percent aluminum, from about 0.05 percent to about 0.6 percent managense, from about 0.025 percent to about 0.3 percent zirconium in solution, up to about 0.035 percent sulfur, up to about 0.01 percent oxygen, residual silicon and phosphorus, and remainder iron except for incidental impurities, and percentages being by weight.
 15. The method of claim 11, wherein said steel initially consists essentially of less than about 0.010 percent carbon, from about 0.05 percent to about 0.6 percent manganese, from about 0.02 percent to about 0.04 percent total aluminum, up to about 0.004 percent nitrogen, up to about 0.035 percent sulfur, up to about 0.01 percent oxygen, residual silicon and phosphorus, from about 0.08 percent to about 0.10 percent total titanium, from about 0.03 percent to about 0.06 percent total columbium, and remainder iron except for incidental impurities, all percentages being by weight.
 16. The method of claim 11, wherein said steel initially consists essentially of less than about 0.010 percent carbon, from about 0.05 percent to about 0.6 percent manganese, from about 0.02 percent to about 0.04 percent total aluminum, up to about 0.004 percent nitrogen, up to about 0.035 percent sulfur, up to 0.01 percent oxygen, residual silicon and phosphorus, from about 0.08 percent to about 0.10 percent total titanium, from about 0.03 percent to about 0.06 percent total zirconium, and remainder iron except for incidental impurities, all percentages being by weight.
 17. The method of claim 11, wherein said atmosphere contains from 3 percent to 6 percent ammonia by volume, and remainder hydrogen.
 18. The method of claim 11, wherein said atmosphere contains ammonia and hydrogen, and the balance an inert gas, with the ammonia to hydrogen ratio adjusted in such manner that the formation of iron nitride or iron-nitrogen austenite is avoided.
 19. The method of claim 11, wherein said heating is conducted at 1,100* to 1,300*F for a period of time inversely proportional to the temperature, and directly proportional to the square of the thickness.
 20. The method of claim 11, wherein said article is deep drawn.
 21. The method of claim 1, including as a final step denitriding said sheet stock in a hydrogen atmosphere at a temperature between about 1,100* and 1,350*F for a period of time sufficient to decrease the ductile to brittle transition temperature to at least about -60*F. 